Several manufacturing processes require the deposition of high quality layers of polymer material onto a substrate. High quality polymeric layers are required for example in semiconductor chip manufacture, where photoresist layers are used in the photolithography process. Another example is the manufacture of flat panel displays, where polymer layers are required for a variety of purposes including arrays of organic light emitting diodes (OLEDs) Yet another example is the manufacture of integrated optical waveguides from polymer materials. The invention will be described with reference to the preparation of integrated optical waveguides, but it will be understood by those skilled in the art to be readily applicable to other manufacturing processes/technologies requiring high quality layers of polymer material.
The term polymer as used herein refers to a molecule of high relative molecular mass, the structure of which comprises portions having multiple repetitions of units derived actually or conceptually from molecules of low relative molecular mass.
The term pre-polymer as used herein refers to any molecule (monomer, oligomer or macromolecule) capable of entering, through reactive groups, into polymerisation or further polymerisation, thereby contributing more than one monomeric unit to the final polymer.
Polymer waveguides are commonly manufactured using a combination of UV, photolithography and etching, where a layer of polymer or pre-polymer (eg. monomers and/or oligomers) is deposited onto a substrate and subsequently etched in a “wet” or “dry” process to pattern waveguides. The etching process is “wet” in cases where the polymer or pre-polymer is hardened by exposure to UV light, so that a suitable solvent can dissolve the unexposed material. Examples of waveguide fabrication via “wet” etching can be found in U.S. Pat. Nos. 4,609,252, 6,037,105, 6,054,253, 6,327,415 and 6,335,149 “Dry” etching includes for example reactive ion etching or plasma etching, where the UV exposure patterns a layer of photoresist on top of the polymer or pre-polymer, rather than the polymer or pre-polymer itself. Examples of waveguide fabrication via “dry” etching are disclosed in U.S. Pat. Nos. 4,824,522, 5,062,680, 5,497,445, and 6,088,492.
Irrespective of the type of patterning and etching processes used, it is necessary to be able to deposit high quality (“optical quality”) layers of the polymer or pre-polymer onto a substrate. If the polymer layer has a rough surface or is of uneven thickness, the resulting waveguides will have excessive scattering loss and may be non-uniform in shape. Several methods are available for depositing layers of polymer or pre-polymer, such as spin coating, dip coating, roller coating, extrusion coating, slot coating, screen printing, meniscus coating, spray coating, curtain coating and doctor blading. Of these, spin coating is widely recognised as the method of choice for depositing optical quality layers.
All of these deposition methods require the polymer or pre-polymer to be in liquid form. Virtually all photoresists and most optical polymers tend to be high molecular weight polymers that are solids or highly viscous liquids, and to facilitate deposition it is necessary to add a solvent to dissolve or dilute them.
In the spin coating process, a pool of material is dispensed onto the centre of the substrate, and the substrate is then spun at high speed (typically 1000 rpm or more) to spread the material into a smooth, thin layer on the substrate. In a common variation, the substrate is initially spun at low speed to spread the dispensed material, then the speed is increased to improve uniformity and produce a thin, highly uniform layer. For successful spin coating, it is vital that the rheology of the liquid be well controlled. It is preferable for the polymer/solvent mixtures used to have low volatility, to minimise changes in the rheology of the polymer solution during the coating process. In particular, any solvent present must have low volatility so that it does not evaporate too quickly during the coating process. Changes in solution rheology during coating can lead to poor quality layers. A “soft bake” step to remove residual solvent is usually necessary prior to any subsequent processing steps.
A known problem with spin coating, discussed for example in U.S. Pat. No. 6,191,053, is that it is extremely wasteful of material, with as much as 90-95% of the liquid dispensed onto the substrate being spun off, and only 5-10% remaining as the deposited layer. Although it is possible to collect and recycle this material, this is generally a difficult and time consuming procedure For applications where high purity is essential, material recycling may be impractical because of the contamination risk. This is especially important in the case of optical waveguide manufacture, where particulate impurities must be minimised to reduce scattering loss. The wastage also represents a major expense that is desirable to avoid.
During optical waveguide manufacture it is usual to produce as many waveguide structures as possible on a single substrate and then dice them out. Such an approach is similar to that used in the semiconductor industry to make electronic integrated circuits. Circular substrates (usually silicon) of varying sizes up to 300 mm in diameter are commonly used in the semiconductor industry and for optical waveguide manufacture. As described in our co-pending application “Methods for fabricating polymer waveguides on large area substrates”, Ser. No. 11/742,194, filed on even date and incorporated herein by reference in its entirety, it is possible to use larger substrates, such as for example 400 mm×500 mm glass or polymer substrates, when manufacturing waveguide devices. Such large area rectangular panels are especially preferred for economy of scale, and also for improved packing efficiency (since many waveguide devices are roughly rectangular in plan view) The flat panel display industry has a range of standard substrates sizes, starting with Generation 1 (270 mm×300 mm) and currently extending to Generation 7 (1870 mm×2200 mm).
However as mentioned above, spin coating is inherently wasteful of material, and when using large area substrates, an alternative deposition technique that is less wasteful of material and yet offers excellent quality layers is particularly desirable. Techniques such as extrusion coating, slot coating, roller coating, meniscus coating, spray coating, curtain coating and doctor blading all have significantly less material wastage than spin coating and can be readily applied to large area rectangular substrates. Extrusion coating in particular, where material is extruded through a nozzle or a slot onto a substrate, has been shown to yield good quality polymer layers, with some manufacturers claiming ±2% thickness uniformity (defined below), as disclosed for instance in U.S. Pat. Nos. 6,495,205 and 6,548,115 However, even this layer quality is not always sufficient for applications such as the manufacture of polymer optical waveguides, where thickness uniformity better than to 5% is generally required. In such situations, a two-stage deposition process may be applicable. Firstly, a low wastage method such as extrusion coating, slot coating, roller coating, meniscus coating, spray coating, curtain coating or doctor blading may be used to deposit a polymer layer as the first stage, and secondly the substrate is spun to further improve the uniformity of the polymer layer Hereinafter, the two-stage deposition process will be referred to as an “extrude-and-spin” process. However it should be understood that, for the purposes of this invention, a two-stage deposition process comprises any liquid layer deposition method as the first stage, followed by spinning as the second stage. That is, where extrusion coating is mentioned, it could equally be some other technique such as slot coating, roller coating, meniscus coating, spray coating, curtain coating or doctor blading
Apart from producing layers with inferior thickness uniformity, a further disadvantage of many low wastage deposition methods, including extrusion coating, is that (compared to spin coating) they utilise a lesser portion of the available substrate area. Typically, an extrusion coater will only deposit material to within 5 mm of the substrate edge, and the edge bead on the deposited material may extend in a further 2 mm, as shown in FIG. 1. In contrast, in spin coating material is deposited light to the edge of a substrate, with an edge bead that may only extend 1 mm in from the edge, as shown in FIG. 2
For the purposes of this specification, thickness uniformity may be defined in a relative manner, as (standard deviation in thickness/average thickness)*100%. However, it will be appreciated that while this commonly used relationship is appropriate for optical layers of conventional thicknesses (up to about 110 μm), it is not entirely appropriate for use in those cases where the layer itself is particularly thick. In very thick layers, good relative thickness uniformity can be obtained even where the variation in surface profile (absolute thickness) is unacceptable.
For the purposes of this specification, thickness uniformity values and/or ranges quoted for layers deposited onto a substrate apply to the entire substrate area except for a 5 mm wide exclusion zone along the periphery of the substrate It will be appreciated by those skilled in the art that irregularities (frequently referred to as “edge beads”) at the substrate edges are difficult to avoid with deposition from the liquid phase. Substrate edges are not usually assessed when considering layer thickness, as edge portions are either generally discarded after dicing (into smaller optical components) or do not form part of the functional region of an optical device.
Unlike in conventional spin coating where spinning can begin as soon as (or a predefined time after) the material is dispensed, with extrude-and-spin there can often be a considerable time lag between the first stage extrusion process and the second stage spinning process An additional time delay is incurred if the extrusion and spinning steps are carried out on different instruments so that the substrate has to be transferred between them Furthermore the extrusion stage takes a finite time, so the material deposited at the beginning of the extrusion stage is on the substrate for a longer time than the material deposited at the end Because of this delay between extrusion and spinning, it is very difficult to accurately and reliably control the layer quality when a solvent-containing polymer material is used in the process, a problem exacerbated in the case of coating large substrates. Not only does the long delay result in evaporation of solvent and any other volatile components, but the variable nature of the delay causes changes in the fluid rheology, and therefore inconsistencies in the final layer properties. Since virtually all photoresists and most optical polymers need to be diluted with or dissolved in a solvent prior to deposition, this is a significant problem for the extrude-and-spin deposition process To prevent such problems, practitioners using conventional solvent-based materials for extrude-and-spin deposition, or for spin coating in general, often must resort to elaborate means such as controlled atmosphere housings with careful control of solvent vapour concentration (U.S. Pat. Nos. 6,238,735; 7,030,039)
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.